With the rapid development and deployment of high-speed railway around the world, it has become imperative for service providers to provide broadband wireless access services for passengers on a train moving at high speed. During a journey up to several hours, the passengers may desire various communication services including voice, email, web browsing, multimedia services, etc. In a typical scenario, a data transmission rate of at least 100 Mbps-1 Gbps would be required in order to provide broadband wireless access to 500-1,000 passengers who require such services on a single high-speed train. This would be challenging for the train's wireless communication system as it is required to support the high-speed movement and the high data rate simultaneously. Further, given the scarcity of spectrum resources, whichever technical standard is adopted for train-track link needs to achieve very high spectrum efficiency to meet the requirement of high transmission rate.
Multiple-input and multiple-output (MIMO) is one of the main technical solutions that improve spectrum efficiency and transmission reliability in current public cellular networks. In general, MIMO multi-stream transmission mode is applicable in cities with strong multi-path transmission effect, especially in an indoor environment, to improve spectrum efficiency because the MIMO channel matrix under such condition is likely to achieve the full rank state. However, in suburbs and in the countryside, the gain of the MIMO spectrum efficiency may be reduced due to the presence of a direct, line-of-sight path between a transmitter and a receiver. This is because the greater correlation between the signals received from different transmission paths causes the rank loss of the MIMO channel matrix and reduces the channel capacity.
There exist techniques that keep the orthogonality between the space sub-channels by optimizing a distance between the MIMO antennas; however, the optimization is achieved based on a certain specific distance between the transmitting antennas and the receiving antennas and an environment with a very high signal-to-noise ratio (SNR) such as 20 dB. Such techniques thus are not suitable for high-speed railway wireless communications because the location of the high-speed train changes rapidly and because of the distance between the transmitting antennas and the receiving antennas and a low SNR.
Moreover, existing techniques utilize a single antenna or a pair of antennas with half-wavelength interval at each trackside base station to provide signal coverage for user devices of users carried by a high-speed train.